There is known an air compressor that detects an air pressure in a tank and restarts a motor thereof when the detected air pressure is equal to or lower than a predetermined value. As a more advanced example, Japanese Patent No. 4,069,450 discloses that an air compressor that detects a change rate of air pressure in a tank and controls a motor according to the detected pressure change rate. This air compressor can be made to operate in a silent mode. In the silent mode, when the detected pressure change rate is equal to or lower than a predetermined value, the motor is restarted.